Butt-in telephone system



June 29, 1948. E. COHRT- 2,444,033

BUTT-IN TELEPHONE SYSTEM Filed May 15, 1943 v v 2 Sheets-Sheet 1INVENTOR 1576/4 coy/Pr AGENT June 29, 1948. E. COHRT BUTT-IN TELEPHONESYSTEI J 2 Sheets-Sheet 2 Filed May 15, 1943 INVENTOR 516/1 (pa /e7-AGENT.

Patented June 29, 1948 UNITED STA-res BUTT-IN TELEEHONE SYSTEMApplication May 15, 1943, Serial No. 487,195 in Denmark August 22, 19407 Claims. 1

The present invention relates to automatic private branch telephoneexchanges in which conversational connections between subscribers takeplace through cord circuits and no busy tone is given when a subscriberis reached who is already connected to. a conversational circuit.

Ordinarily, a. caller selecting a busy subscriber is not admitted onthis subscribers connection but receives a busy tone. In P. B. Xs forquite special purposes it is necessary, however, that a connection isalways established immediately. In such case, it is desirable to send tothe subscribers carrying on the original conversation or signalindicating that still another subscriber has joined the connection. Itis thus possible for a comparatively great number of subscribers tosimultaneously carry on a conversation with one another. P. B. Xs thuspresent problems which do not exist in ordinary systems.

.An object of the present invention is to provide a P. B. X circuit inwhich a subscriber can be released. without other subscribers, talkingover the same cord circuit, being released.

Another object of the invention is to provide means whereby asubscri'bercan be released from the connection over which he originally talked andcontinue the conversation with a new :subscriber connected to his line.

In brief I accomplish these and-other objects of the present inventionas will become more apparent hereinafter, in the following manner:

Thecord circuits according to the present invention include a device bymeans. of which a cord circuit at call from a subscriber, who wants totalk to subscriber who is engaged by a conversational connection throughanother cord circuit, is connected to this latter connection and movesto listening-in position. In the latter position, a warning tone isflashed to the busy subscribers, and the same is used to lock theoriginal cord circuit as long as at least one of the subscribers who wasoriginally connected to it, is engaged by a conversational connection.

The invention will be more readily understood from the followingdescription thereof and the accompanying drawings which illustrate anembodiment of the invention, by way of example. In the drawings- Fig. 1illustrates a subscriber line circuit and a starting circuit;

Fig. 2 illustrates a cord circuit according to the invention; and

Fig. v3 illustrates a circuit for unused lines.

In the subscriber line circuit and starting circuit shown in Fig. 1,each subscriber line is provided with a combined line and cutoff-relayLr, whereas a pilot relay Sr and a test resistance are common.to a maximum of twenty line circuits.

Astarting relay Urr is common toall the subscriber lines of the unit.When a subscriber desires to make a call and lifts the telephone off thecradleat his station, .a line loop is closed, and relay Lrtis half.attracted and .closes its right middle front contact; it is notsufiiciently energized for breaking its other contacts.

Thepilot relay Sris enengizedfollowed by relay Urr which supplies groundconnection to the starting relays in the cord circuits.

The calling line is characterized by testpotential on the contact c(Fig. .2) of the line finder arc. All idle line finders rotate, and whenone of these has found the calling .line it stops, and direct groundconnection isgiven from the cord circuit .to the 0 wire, whereby relayL1. is energized strongly, so that. it. is entirely energized andseparates itself from the a and b wires, after which the relays Sr andU11 are released, and the other rotating line finders stop. The testbattery is interrupted at the right innermost back contact of .relay Lr.

When a line selector at call to a subscriber is adjusted to asubscriber, direct ground is connected to the 0 wine, so that relay L7.is energized entirely and is interrupted from the a and 1) wires.

The cord circuit shown in Fig. .2 consists of a 50 point line finderSIVDF, one, or possibly two line selectors SMSI and SMSZ, and a group ofrelays. The exchange has a capacity of '50 subscribers, when the lineselector SMS] is applied alone, while a capacity of 99 subscribers isobtained when the extra line selector SMS2 is applied too.

Connections to idle subscribers are established in the ordinary way, butno busy tone is given in case of a call to a busy subscriber. In thisevent, the connection is established immediately after the warning tonehas. been sent which indicates to the talking subscribers that a thirdparty has just joined the connection.

The cord circuit, over which the original conversation was established,is thereupon lockedso long as a conversation is carried on with theoriginally calling or originally called subscriber, or possibly both.The other cord circuits, over which connection is established to a busyline, move to listening-in position, so that microphone feeding currentis supplied from the original cord circuit-only. I

When a calling subscriber lifts the telephone off the cradle, the linerelay is energized and gives starting potential to the starting relaysLsr of the cord circuit, which relays energize over a jack Ba andbattery on the outermost left back contact of relay Ar. The line finderSMF rotates by means of the following circuit: battery, a resistance,the magnet SMFM, the interrupter of this magnet, back contact of relayDr, a front contact on relay Lsr, a back contact on relay Pbr, a backcontact on relay Ltr, a normal contact on a selector SMSZ, normalcontact on the selector SMSI, the a are of selector SM in normalposition, back contact on relay Ar, back contact on relay Asr andground.

The line finders SMF in all idle cord circuits rotate and hunt for thecalling subscriber who is characterized by test potential on the econtact of the line finder arc, to which the test relay Ltr is connectedover the outermost left front contact of relay Lsr.

When the line finder reaches the contact corresponding to the callingsubscriber, the test relay is energized and interrupts the circuit forthe line finder magnet, so that the selector SMF stops. Via the frontcontact of relay Ltr and the back contact of relay Tcr as well as theinnermost right front contact of relay Lsr ground connection isestablished through an 1800 ohms resistance on relay Bar to relay Br,which is energized slowly in order to prevent two cord circuits frombeing connected simultaneously to the same subscriber, relay Pbr beingenergized via the outermost right front contact of relay Lsr. Relay Pbrconnects ground to the 6 ohms winding of relay Ltr, whereby doubleconnection is prevented in known way, because two relays Ltr cannot beenergized simultaneously when their 6 ohms windings are inserted.

When relay Br closes its front contact, a circult is closed for relayDr, which is attracted and through-connects the a and 1: wires to afeeding relay Asr which is energized, while at the same time ground isconnected to the wire of the subscriber line via the 0 brush of the linefinder, whereby the line relay is entirely energized and switches offthe subscriber line and interrupts the starting ground and the testbattery. All the other rotating finders stop and from a front contact onrelay Dr a circuit is closed for relay Ar which is energized. Relay Arinterrupts the battery from relay Lsr, which, however, is alreadyreleased, so that it can no longer be energized; relay Ltr was releasedtogether with relay Pbr when the test battery was interrupted, whereasrelay Br is kept energized through a resistance of 2000 ohms on relayHar from the front contacts of relay Asr. Relay Cbr is energized overthe outermost left front contact of relay Har from ground through apilot relay in the ringingand tone-circuit; the starting of the tonesignal is obtained via the front contact of relay Tor.

The following circuit is closed for relay I-Isr: battery, left outermostfront contact on relay Ar, front contact on relay Cbr, a resistance,winding on relay Hsr, back contact on relay Ftr, back contact onrelayOtr, back contact on relay Rcr, right innermost front contact onrelay Ar and ground.

Relay Cbr is then kept energized over the innermost left front contactof relay I-llsr from ground in the ringingand tone-circuit in parallelconnection with relay Tor, which is energized over another front contactor relay Hsr. The 2000 ohms resistance on relay Har is Short-circuitedover the right innermost front contact of relay Hsr, so that relay Br islocked direct from the front contact on relay Asr, whereby it issaturated and hence slow in releasing.

Dialing tone is sent to the calling subscriber in the following circuit:dialing tone generator, left front contact on relay Br, Te M1 condenser,the b are of the selector SM in normal position, left middle frontcontact on relay Ar, uppermost back contact on relay Ftr, a condenserand the a wire of the subscriber line.

The subscriber dials the first digit of the wanted number and relay Asrtransmits the impulses via its back contact to the magnet SM in thefollowing circuit: ground, back contact on relay Asr, left innermostfront contact on relay Hsr winding on relay Lmr, back contact on relayTcr, winding on the magnet SMM, a resistance, battery.

Whenthe selector SM has made the first step the dialing tone isinterrupted.

Relay Lmr is energized during the impulsing, and closes via its frontcontact the following circuit for relay Har: battery, winding on relayHar, front contact on relay Lmr, back contact on relay Tcr, back contacton relay Lsr, innermost left front contact on relay Har and to ground.

Relay I-Iar is energized and locked to ground on relay Dr over itsmiddle left front contact,

after which it interrupts its own magnetizing circuit at its innermostback contact.

Relay Ltr is energized after the release of relays Lsr and Pbr in thefollowing circuit: ground, a winding on relay Ltr, back contact on relayPbr, front contact on relay I-Isr, marking line l, are c of selector SMin normal position, a resistance, and battery.

When the first impulse has been received and the selector SM has madeone step, relay Har is energized as already mentioned, and the followingcircuit is closed for the magnet SMSIM: battery, a resistance, themagnet SMSIM, its interrupter, right innermost back contact on relaySdr, outermost right front contact on relay Har, outermost right backcontact on relay Tcr, back contact on relay Ltr. 'I-Iowever, inasmuch asrelay Ltr is energized, as already mentioned, the selector SMSI rotatesonly if the first digit consists of two or more impulses.

As soon as the selector SM releases the marking lin No. 1, the circuitfor the magnet SMSlM is closed, whereby the selector SMSI rotates. Whenthe first digit is received, relay Asr remains energized, whereas relayLmr is released.

When the selector SMSI reaches the marking line on which the selector SMis standing, a circuit is closed for relay Ltr via the e are and e brushof the selector SMSI, and this relay is energized and stops theselector. The following circuit is closed for relay Tcr after therelease of relay Lmr: ground, right middle front contact on relay Ar,front contact on relay Ltr, back contact on relay Tcr, back contact onrelay Imr, front contact on relay Har, winding on relay Tcr and battery.

Relay Tcr is energized and locked via its outermost left front contactto ground on relay Ar and opens at its outermost right front contact thecircult for the selector SMSI, so that the finder SSI cannot rotate anymore.

If the capacity of the exchange exceeds 50 lines, relay Sdr is energizedby the tenth digits 6, 7, 8, 9, 0 via the d arc of the selector SM andchan es over from the line selector SMSI to SMSZ.

ascr bes Thesubscriber. now selects the units digit and relay Asroperates in accordance with the dial. The: impulses are transmitteddirect to the magnet of the line selector viathe following circuit:ground, back contact on relay Asr, front contact onrelay'Hsr, winding onrelay Imr, front contact on relay Tcr, contact on relay Sdr, winding onthe magnet SMSIM, 48 ohms resistance and battery. -Relay Lmr isenergized during the .impulsing and closes the following circuit forrelay Pbr: battery, winding on relay Pbr, left innermost front contacton relay Tcr, front contact onrelay Lmr, left middle front contact onrelay Har, frontcontact on relay Dr and to ground.

Relay Pbrcloses the following circuitfor relay Rcrtbattery; winding onrelay Rcr, left middle front contact on relay Pbr, back contact on relayFtr, back contact on relay Otr, back contact onrelay Rcr, rightinnermost front contact on relay Ar and to ground.

Relay Rcr is locked via its own front contact to ground via the backcontacts of relays Otr and Ftr and interrupts its own magnetizingcircuit. Relay Ltr is released when the selector SMS'l leaves themarking line.

When the units digit has been received, the selector SMSI is adjusted tothe line of the wanted subscriber, and relay Asr remains energized,whereas relay Lmr is released.

When relay Lmr is released after the last digit has been received, relayPbr is released (which was locked'over the front contact of relay Lmr)and the following circuit is closed between the line of the calledsubscriber and relay Ftr: battery, winding on relay LT (line circuit), 0line, the 0 brush of the line selector, front contact on relay H windingon relay Fir, front contact on relay 'Icr, left front contact on relayCbr, right innermost back contact on relay Pbr, back contact cnrelayLtr, right middle front contact on relay A and to ground.

It should be noted that when relay R0?" was energized, the energizingcircuit for relay Hsr was interrupted, this relay re-mains, however,energized via the left middle front contact of relay Pbr in parallelconnection with relay Tcr during the impulsi-ng. After the release of'relay Pbr, the circuit for relay I-Isr is interrupted,

but since this relay releases slowly, it allows relay Ftr to energize incase of connection to an idle subscriber via its innermost right frontcontact. When relay Ftr closes its uppermost front contact it is lockedin following circuit: battery, winding on relay Fir, front contact onrelay Fir, winding on relay Ftr, front contact on relay Tor, frontcontact on relay Cbr, back contact on relay Pbr, back contact on relayLit, right middle front contact on relay Ar and ground.

Relay Lr of the subscriber line is energized in series with relay Ftr,and when relay Hsr closes its right innermost back contact it remainsenergized from direct ground connected to the 0 line in followingcircuit: battery, a wmding on relay LT (line circuit), the c brush ofthe line selector, right innermost back contact on relay Hsr, lowestfront contact on relay Fir, right front contact on relay Cbr and toground.

After the release of relay Hsr relay Chi is un der control of relay Rcrvia the lowest front contact of this latter'relay.

Ringing signal is sent to the called subscriber via following circuit:ringing generator, front contact on relay Rc'r, winding on relay Csr,right innermost back contact on relay Otr, lowest front 15 contactonrelay Ftr, biline, thesubscrlber apparatus, a line, uppermost frontcontact on relay Fir, lowest front contact on relay R01, innermost rightfront contact on relay Ar and ground.

The 750 ohms' winding of relay Csr is shunted by a winding on relay Rcr,whereas itsother winding is sh-ort-circuited by ground on the a wire, sothat relay Csr is made slow in becoming energized.

Callback toneis sent to the calling subscriber from the left outermostback contact of relay PM 'via' a front contact on relay R011 and the 17wire.

When the called subscriber lifts the telephone off the cradle, the lineloop is closed, whereby relay Csr receives sufiicient current throughits right winding for energization, and since relay Rcr is locked via aback contact on relay Csr, this relay is; released immediately andinterrupts the ringing and the call back tone and throughconnects theconversational connection viatw condensers.

Relay Cs'r remains attracted via the a and b wires in series with thecalled subscriber apparatus and provides microphone feeding current. Theconversational circuit is now closed between the calling and the calledsubscriber.

Relay Cbr, the magnetization circuit of which is to be interrupted byrelay Bar, is then kept energized from the left front contact of relayCsr. When relay Bar is released the connection to theringingandtone-circuit is interrupted by relay Rcr. and this circuit stops. 1

During the conversation the cord circuit is under control of the callingas well as the called subscriber. If the calling subscriber restores thetelephone handset to the cradle at his station, thereby normallydisconnecting or d sengaging the station, the cord circuit is released,and the calling as well as the called subscriber are releasedimmediately. In. case th called subscriber restores his telephone first.he releases himself immediately. whereas the cord circuit is notreleased until the calling subscriber restores his telephone.

When the calling subscriber restores his telephone relay Asr is releasedso that relay Br, which after the release of relay Hsr was lookedthrough a 2000 ohms resistance on relayHar, is released and interruptsthe ground connection on relay Dr which is released and interrupts thesub- .scriber line from the cord circuit. Relay Dr interrupts thelocking circuit for relay Ar which is released and interrupts thelocking circuit for relay Tor which also releases. Relay Fir is releasedtogether with all the other relays in the circuit.

The selector SM rotates to normal position by a circuit Via its contactarc a and its interrupter. When it reaches normal position a circuit isclosed for the line selector via its home contact and interrupter, andthe line selector moves to normal position, after which the cord circuitis I released and ready for new calls.

When the called subscriber is released. relay Csr is released, the lineloop is interrupted, and the circuit for relay Clir is interruptedsothat this relay is released slowly. Ground on the 0 wire, which kept theline relay energized, is in terru-pted and the line becomes idle. At thesame time the locking circuit for relay Ft-r is opened bythe leftfrontcontact of relay CM, and relay Ftr is released and disconnects thecalled subscriber from :the cord circuit. The cord.- circuit itself isnot released until the calling subscriber restores the telephone asexplained above.

In case of a call to a busy subscriber, relay Ftr cannot attract thearmature when the wire is being tested, a direct ground connection beingfound from the cord circuit, over which the original conversation wasestablished.

Relay Ftr then remains in normal position after the units digit has beensent, and relay Hsr is released. Relay Tor is released slowly after therelease of relay I-Isr and a warning tone is sent during the releasingtime'of relay T01- to the talking subscriber via the following circuit:dialing tone generator Sg, left front contact on relay Br, condenserright outermost back contact on relay Hsr; left front contact on relayTor and the bbrush of the line selector.

This tone informs the talking subscribers that athird subscriber hasjoined the connection.

The following circuit is closed for relay Otr which is slow in gettingenergized: battery, winding on relay Otr, left innermost front contacton relay Tc'r, back contact on relay Tor, lowest back contact on relaFtr, front contact on relay Cbr and to ground.

Relay Otr is energized and interrupts the locking circuit for relay Rcr,which is released and interrupts the feeding relay Csr from the a and bWires. The calling subscriber is now through connected to the busysubscriber via the innermostfront contacts of relay 0151', so that thecalling subscribers may now talk to the busy subscriber. Relay Cbr iskept energized via a front contact on relay Otr and a back'contact onrelay Csr. Via the right outermost front contact of relay Otr ground isconnected to the (1 brush of the line selector, and via the d multiplewire this ground is carried to the cord circuit, over which the originalconversation was established, and maintains it, so that it cannot bereleased which must not happen, since the microphone current is fed fromthere.

When the calling subscriber is released the cord circuit is released asalready mentioned, and rela Otr interrupts the ground connection fromthe d multiple wire, so that the busy cord circuit is released in normalway.

During a normal conversation with an idle sub scriber, the relays KOTland KO'Iz are connected via the relays Dr and Ftr, respectively, to thed brushes of the line finder and the line selector. In case of a call toa, busy subscriber the cord circuit, which went to listening-inposition, as explained above, connects ground to the d multiple wire, sothat relay Kori is energized, in case the call is to a callingsubscriber, and relay Ken is energized, in case the call is to a calledsubscriber.

At call to a callingsubscriber, relay Kori is energized as explainedand. connects ground to the relays Ar and Dr, which are in this wayunder control of the cord circuit in listening-in position, and the cordcircuit is, therefore, locked as long as conversation takes place to thecalling subscriber.

- At call to 9. called subscriber, relay Korz is energized and connectsground to relay Ar, which is locked and locks the cord circuit.

At call to a calling subscriber, where the called subscriber isreleased, the cord circuit is locked via relay KOTI, and when thecalled'subscriber in the original conversation restores his telephone,relay Csr is released followed by relay Clrr.

.Relay Cbr interrupts the ground from the c line of the calledsubscriber, and relay Ftr is released, when relay C'b-r interrupts itsleft front contact, so that the called subscriber is free.

At call to a called subscriber, where the calling subscriber isreleased, relay Asr is released followed by relay Br. Relay Dr'isreleased and interrupts the calling subscriber from the cord circuit,whereas the cord circuit (as aforementioned) is locked from the frontcontact of relay KOTz. When the conversation is finished, relay KOTz isreleased and interrupts the connection to relay Ar, which isreleased andreleases the cord circuit.

On simultaneous calls to the calling as well as the called subscriber,relay Kan as well as Korz are energized. If it is desired to interruptthe original conversation and the called as well as the callingsubscriber want to continue with the new conversation, they mayinterrupt the existing connection by depressing the cradle for a moment.When the calling subscriber depresses the cradle relay Asr is releasedfollowed by relay Br, but, since relay Kori, is energized, relay Drcannot be released. Via the back contact of relay Br and the frontcontact of relay K07). a circuit is closed for relay T'br, which isenergized and locks. The relay Asr and Br are energized a moment later.The original conversation is now interrupted by relay Tbr, and feedingcurrent is supplied from the relays Asr and Csr to the calling as wellas the called subscriber.

When the called subscriber depresses the cradle, relay Csr is released,followed by relay Cbr. Relay Ftr is not released, since it is keptenergized via the lowest front contact of relay KOTz. At the same time,relay K012 replaces the ground on the outermost right front contact ofrelay Cbr, so that the line relay is kept energized. The followingcircuit is closed for relay Tbr: battery, winding on relay Tbr, lowestfront contact on relay Kor, left back contact on relay Cbr, back contacton relay Pbr, back contact on relay Ltr, middle right front contact onrelay Ar and to ground.

Relay Tbr interrupts the talking circuit between the calling and thecalled subscriber. When relay Csr energizes again, it is followed byrelay Cbr.

The cord circuit will not be able to be released until relay KOTI aswell as Kora are released, i. e., when conversations are no longertaking place to the calling or called subscriber.

It should be noted that when the conversation to the calling or calledsubscriber is interrupted, he will be released from the cord circuit asabove explained.

In case a tenth digit is dialed, which is not used for subscribernumbers, a busy tone is given via the b are of the selector SM to thecalling subscriber.

Fig. 3 shows a circuit for unused lines. 7

In the usual telephone systems an unused line is characterized by aninterrupted c line on the c are of the line selector, which causes thecord circuit to send a busy tone. In the P. B. X without test no busytone is sent, for which reason it is necessary to connect a circuit forunused lines to the line selector arc and interrupt the normal lineequipment belonging to unused lines.

When a call is made to an unused line, relay Cor is energized and startsthe tone. The cord circuit moves to ringing position, but due to thepresence of a 500 ohms induction coil, it instantly moves to talkingposition, after which the busy tone is sent via the front contact ofrelay Cor t0 the calling subscriber, who then restores the telephone andreleases the cord circuit.

It will be clear that many changes may be made in the construction andarrangement described above without departing from the spirit and scopeof the present invention. I therefore do not wish to be understood aslimiting myself to the details of construction and arrangement set forthherein.

I claim 1. An automatic private branch telephone exchange of the typepermitting a conversational connection between a first and secondsubscriber, and in which a third subscriber can be connected to thefirst or second subscriber without receiving a busy signal, being thusenabled to participate in the conversation, comprising in combination aplurality of cord circuits, a telephone subscribers station associatedwith each cord circuit, first means for interconnecting at least a pairof said stations over the respective cord circuits to establish aconversational connection, second means for interconnecting at least oneof the remaining cord circuits to said conversational connection forparticipation in any conversation over said conversational connection,control means operatively connected to said second means for sending aWarning signal to the receivers connected to the original conversationalconnection when one of said remaining cord circuits is connected to saidconversational connection, said contnol means also locking said originalconversational connection as long as one of the stations of the originalconversational connection remains connected, whereby the subscribers onthe original conversational connection are advised by said warningsignal when a new subscriber cuts himself in, and conversation with suchnew subscriber may be continued, due to said locking action, as long asone of the subscribers on the original conversational connection remainsin the circuit, though the other cord circuits are interrupted.

2. The system claimed in claim 1, in which said control means includescoupling means actuated by said second means when one of said remainingcord circuits is connected to said conversational connection, wherebysaid conversational connection is looked as long as conversation takesplace over said connected remaining cord circuit.

3. The system claimed in claim 1, in which said control means includes arelay actuated by said second means when one of said remaining cordcircuits is connected to said conversational connection, whereby saidconversational connection is looked as long as conversation takes placeover said connected remaining cord circuit.

4. The system claimed in claim 1, comprising a relay connected to saidconversational connection and actuated by manual disengagement of one ofthe subscribers stations thereof, said relay being adapted to releasesaid disengaged station from its cord circuit.

5. The system claimed in claim 1, in which said conversationalconnection comprises two connected cord circuits, a first and secondsubscribers station associated with said two cord circuits, a firstrelay connected. to and actuated by said first receiver, said firstrelay being adapted to disconnect the cord circuit appertaining to saidfirst subscribers station, when the latter is manually disconnected; asecond relay connected to and actuated by said second subscribersstation, said second relay being adapted to disconnect the cord circuitappertainin to said second subscribers station when the latter ismanually disconnected.

6. The system claimed in claim 1, in which said control means includes arelay actuated by said second means when one of said remaining cordcircuits is connected to said conversational connection, whereby saidconversational connection is looked as long as conversation takes placeover said connected remaining cord circuit and in which saidconversational connection comprises two connected cord circuits, a firstand. a second subscribers station associated with said two cordcircuits, a first relay connected to and actuated by said first station,said first relay being adapted to disconnect the cord circuitappertaining to said first station, when the latter is manuallydisconnected; a second relay connected to and actuated by said secondstation, said second relay being adapted to disconnect the cord circuitappertaining to said second subscribers station when the latter ismanually disconnected, all said relays being connected in such a waythat upon a simultaneous call to said first and second stations, whilethe original conversational connection still subsists, the subscriber atsaid first or second stations may interrupt said original conversationand continue with the new one by momentarily disconnecting therespective station.

7. The system claimed in claim 1, comprising signalling means associatedwith said cord circuits, said signalling means bein adapted to send abusy signal over a cord circuit when a subscriber in said circuitselects a non-existing number.

EIGIL COHRT.

REFERENCES CITED FOREIGN PATENTS Country Date Denmark Aug. 2'7, 1945Number

